54 CHEMICAL SENSORS: TECHNOLOGIES. VOLUME 4: SOLID-STATE DEVICES (MOXs) (Azad et al. 1992). In contrast to other semiconductors that undergo irreversible chemical reac- tions upon exposure to air at high temperatures, the MOXs remain stable while interacting with oxygen or hydroxyl groups adsorbed at their surfaces. This property provides reliable operation of correspond- ing devices even in rather aggressive media. In addition, the metal oxides meet the specific requirements that sensor materials have high chemical, mechanical, and thermal stability. Another key feature which makes MOXs attractive is that their semiconducting properties are rather easily tuned during fabrication and/or by adjusting operating conditions (Sukharev and Myasnikov 1986). In this chapter we address the following issues related to such sensors: (1) fundamentals, (2) construction designs, (3) fabrication routes, and (4) approaches to optimizing their parameters for application. 2. FUNDAMENTALS OF GAS SENSING EFFECTS IN METAL OXIDE­ BASED SENSORS: MAIN PRINCIPLES OF METAL OXIDE GAS SENSOR OPERATION The operation of MOX-based gas sensors is usually described in terms of two frameworks (Heiland 1982; Gopel 1991). The first considers mainly the space charge effects/changes of the electric surface potential due to surface reactions such as "ionosorption" of gaseous molecules, chemical reactions, and catalysis. The second framework considers the sensing effects via changes in the bulk MOX stoichio-